1. Field of the Invention
This invention relates to a micro cold cathode which may be used as an electron beam source for a variety of electron beam devices such as a flat panel display and a CRT.
2. Description of the Related Art
Recently, a field emission type of cold cathode has been intensively studied and developed, in which a conductive substrate, an insulating layer, a gate electrode layer and a cathode emitter with a sharp tip within the openings thereof may be formed as an integrated part using a semiconductor fine processing technology. Such a cathode is expected to be applied to a high-performance electron gun.
A typical example of a manufacturing technique for a conventional field emission type of cold cathode may be a Spindt-type cold cathode, whose manufacturing process is shown in FIG. 5.
First, on a silicon substrate 1 are sequentially deposited a silicon oxide film 2 and a gate film 3(FIG. 5(a)), and after forming a resist 4, a circular opening is formed via etching(FIG. 5(b)). Then, a sacrificing layer 5 consisting of aluminum is formed by oblique vapor deposition, rotating the substrate(FIG. 5(c)). After forming the sacrificing layer 5, an emitter material is deposited from a vertical direction to the substrate by means of an appropriate procedure such as a vapor deposition technique under a high vacuum. As the emitter material is deposited, the emitter material is gradually condensed around the opening of the sacrificing layer, leading to reduction of the opening diameter and then blockage of the opening. At the same time, a conical emitter is formed on the conductive substrate(FIG. 5(d)). Finally, the sacrificing layer 5 and the emitter material layer 6 deposited thereon are simultaneously etched off to give a cold cathode(FIG. 5(e)).
A process for manufacturing a silicon cold cathode will be described by referring to FIG. 6. First, a silicon mask 20 is formed on a predetermined region of a silicon substrate(FIG. 6(a)). The substrate is then subject to isotropic etching(FIG. 6(b)) and then oxidation to form a conical emitter(FIG. 6(c)). Then, a silicon oxide film 2 and a gate film 3 are sequentially deposited on the whole surface of the substrate(FIG. 6(d)). Finally, the silicon oxide film 2 is etched off to give a cold cathode(FIG. 6(e)).
The cold cathode with either of the above structures, however, has a problem that during its use and/or voltage of application, the conical emitter may be sputtered by suspended metal particles and/or ion bombardment, resulting in adhesion of a conductive film on the side of the insulating film (silicon oxide film). Thus, current leak may occur between the emitter and the gate, leading to destruction of the cold cathode.
Hence, JP-A 8-321255 has suggested the following structure for ensuring insulation between an emitter and a gate.
FIG. 3 shows an example, where an insulating layer is etched for keeping the side of the insulating film away from the opening for the emitter and the gate, to avoid adhesion of, for example, suspended metal particles on the side of the insulating film.
FIG. 4 shows another example, where two types of insulating films with different etching rates during formation of the insulating film, i.e., the first and the second insulating films 10, 11, are alternately formed, which provides a corrugated shape after etching. It intends to increase a creeping distance on the side face for preventing current leak.
These cold-cathode structures for preventing current leak, however, cannot adequately prevent current leak. Thus, for adequately preventing the leak, it is necessary to increase the etching amount of the insulating film in the transverse direction. However, it may cause increase of a pitch between emitters in an emitter array, leading to decrease of a current density.
This invention for solving the above problems provides a micro cold cathode comprising a substrate having at least one conductive surface, an insulating film formed thereon and a conductive gate film formed thereon, in which there is formed an opening reaching the substrate in the insulating film and the conductive gate film and an emitter electrode is formed in the opening, wherein there is provided a shield member which spatially shields at least part of the insulating film from the emitter electrode.
This invention further provides a micro cold cathode comprising a substrate having at least one conductive surface, an insulating film formed thereon and a conductive gate film formed thereon, in which there is formed an opening reaching the substrate in the insulating film and the conductive gate film and an emitter electrode is formed in the opening, wherein there is provided a shield member which spatially shields at least part of the insulating film from metal particles emitted from the emitter electrode during voltage application.
The cold cathode of this invention has a shield member between the side of the insulating film and the conical emitter to spatially shield at least part of the insulating film from the emitter electrode. Thus, it can effectively prevent, for example metal particles from the emitter electrode from adhering to the side of the insulating film, which may cause forming a conductive film on the side, and thus can prevent current leak. Here, the shield member may be a wall or step formed between the emitter electrode and the insulating film.
This invention further provides a process for manufacturing the above micro cold cathode having a shield member between the emitter electrode and the insulating film, comprising the steps of;
(a) forming the first insulating film on a silicon substrate, and then a groove on the first insulating film;
(b) forming the second insulating film on the upper face of the first insulating film, filling the groove;
(c) forming a gate film on the second insulating film;
(d) forming an opening reaching the substrate so that the sides of the first insulating film, the second insulating film and the gate film are exposed; depositing a sacrificing layer around the opening from an oblique direction; depositing an emitter material to form a conical emitter; and then etching the sacrificing layer off; and
(e) wet-etching the first and the second insulating films to form a wall around the emitter. In the above step (c), a silicon nitride film may be formed between the second insulating film and the gate film. It may allow a creeping distance to be increased and thus may prevent leak between the lower surface of the gate film and the substrate.
This invention further provides a process for manufacturing the above micro cold cathode having a shield member between the emitter electrode and the insulating film, comprising the steps of;
(a) forming a nitride film and the first insulating film on a substrate, applying a photoresist to the region of the nitride and the first insulating film, and then forming a mask for emitter formation by etching the nitride and the first insulating films using the photoresist;
(b) forming a cylindrical structure by etching the silicon substrate using the mask for emitter formation;
(c) oxidizing the substrate to form a narrowed region in the center part of the cylindrical structure by means of a silicon oxide film;
(d) conducting silicon-oxide anisotropic etching and silicon anisotropic etching using the nitride film as a mask to form a silicon cylindrical structure below the narrowed region, leaving the silicon oxide film under the nitride film;
(e) oxidizing the silicon cylindrical structure until the silicon part is divided into two subparts at the narrowed region;
(f) etching off the nitride film and the silicon subpart above the narrowed region, and then forming a groove on the first insulating film;
(g) forming the second insulating film on the upper face of the first insulating film, filling the groove;
(h) etching off the gate film and the third insulating film to expose the side of the gate film; and
(i) dry-etching the silicon nitride film and wet-etching the second and the first insulating films, to expose the emitter tip and form a step of the first insulating film.
According to the process for manufacturing a micro cold cathode of this invention, a shield member can be suitably provided around the emitter electrode, utilizing a groove formed on the first insulating film.
In the above process of this invention, it is preferable that the first insulating film has a lower etching rate during wet-etching than that of the second insulating film because it allows a wall or step to be readily formed. Typical combinations of the first and the second insulating films include thermal oxide and HTO(High Temperature Oxide) films, as well as silicon nitride and silicon oxide films, respectively.
As described above, the micro cold cathode of this invention has a wall or step between the side of the insulating film and the conical emitter to effectively prevent a conductive film from being deposited on the side of the insulating film and thus to effectively prevent current leak.